X-ray absorption Fe-K edge data on ferrous and ferric model complexes have been studied to establish a detailed understanding of the 1s f 3d pre-edge feature and its sensitivity to the electronic structure of the iron site. The energy position and splitting, and intensity distribution, of the pre-edge feature were found to vary systematically with spin state, oxidation state, geometry, and bridging ligation (for binuclear complexes). A methodology for interpreting the energy splitting and intensity distribution of the 1s f 3d pre-edge features was developed for highspin ferrous and ferric complexes in octahedral, tetrahedral, and square pyramidal environments and low-spin ferrous and ferric complexes in octahedral environments. In each case, the allowable many-electron excited states were determined using ligand field theory. The energies of the excited states were calculated and compared to the energy splitting in the 1s f 3d pre-edge features. The relative intensities of electric quadrupole transitions into the manyelectron excited states were obtained and compared to the intensity pattern of the pre-edge feature. The effects of distorting the octahedral iron site to tetrahedral and square pyramidal geometries were analyzed. The contributions to the pre-edge intensity from both electric quadrupole and electric dipole (from 3d-4p mixing) intensity mechanisms were established for these distorted cases; the amount of 4p character and its distribution over the many-electron final states were experimentally estimated and compared to theoretical predictions from density functional calculations. The methodology was also applied to binuclear complexes, and a clear marker for the presence of a µ-oxo Fe-O-Fe bridge was established. General trends in 3d-4p mixing are developed and discussed for a series of geometries and oxidation states of Fe complexes. The results presented should further aid in the interpretation of the 1s f 3d pre-edge region of iron complexes and non-heme iron enzymes.

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Abstract:Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand π* orbitals due to metalto-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of π back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.